userns: Add a limit on the number of user namespaces

Export the export the maximum number of user namespaces as
/proc/sys/userns/max_user_namespaces.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2016-08-08 13:41:24 -05:00
parent dbec28460a
commit b376c3e1b6
4 changed files with 81 additions and 11 deletions

View File

@ -44,9 +44,15 @@ struct user_namespace {
struct ctl_table_set set;
struct ctl_table_header *sysctls;
#endif
int max_user_namespaces;
atomic_t user_namespaces;
};
extern struct user_namespace init_user_ns;
extern bool setup_userns_sysctls(struct user_namespace *ns);
extern void retire_userns_sysctls(struct user_namespace *ns);
extern bool inc_user_namespaces(struct user_namespace *ns);
extern void dec_user_namespaces(struct user_namespace *ns);
#ifdef CONFIG_USER_NS

View File

@ -321,6 +321,8 @@ void __init fork_init(void)
init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
init_task.signal->rlim[RLIMIT_SIGPENDING] =
init_task.signal->rlim[RLIMIT_NPROC];
init_user_ns.max_user_namespaces = max_threads;
}
int __weak arch_dup_task_struct(struct task_struct *dst,

View File

@ -43,7 +43,18 @@ static struct ctl_table_root set_root = {
.permissions = set_permissions,
};
static int zero = 0;
static int int_max = INT_MAX;
static struct ctl_table userns_table[] = {
{
.procname = "max_user_namespaces",
.data = &init_user_ns.max_user_namespaces,
.maxlen = sizeof(init_user_ns.max_user_namespaces),
.mode = 0644,
.proc_handler = proc_dointvec_minmax,
.extra1 = &zero,
.extra2 = &int_max,
},
{ }
};
#endif /* CONFIG_SYSCTL */
@ -55,6 +66,8 @@ bool setup_userns_sysctls(struct user_namespace *ns)
setup_sysctl_set(&ns->set, &set_root, set_is_seen);
tbl = kmemdup(userns_table, sizeof(userns_table), GFP_KERNEL);
if (tbl) {
tbl[0].data = &ns->max_user_namespaces;
ns->sysctls = __register_sysctl_table(&ns->set, "userns", tbl);
}
if (!ns->sysctls) {
@ -78,6 +91,46 @@ void retire_userns_sysctls(struct user_namespace *ns)
#endif
}
static inline bool atomic_inc_below(atomic_t *v, int u)
{
int c, old;
c = atomic_read(v);
for (;;) {
if (unlikely(c >= u))
return false;
old = atomic_cmpxchg(v, c, c+1);
if (likely(old == c))
return true;
c = old;
}
}
bool inc_user_namespaces(struct user_namespace *ns)
{
struct user_namespace *pos, *bad;
for (pos = ns; pos; pos = pos->parent) {
int max = READ_ONCE(pos->max_user_namespaces);
if (!atomic_inc_below(&pos->user_namespaces, max))
goto fail;
}
return true;
fail:
bad = pos;
for (pos = ns; pos != bad; pos = pos->parent)
atomic_dec(&pos->user_namespaces);
return false;
}
void dec_user_namespaces(struct user_namespace *ns)
{
struct user_namespace *pos;
for (pos = ns; pos; pos = pos->parent) {
int dec = atomic_dec_if_positive(&pos->user_namespaces);
WARN_ON_ONCE(dec < 0);
}
}
static __init int user_namespace_sysctl_init(void)
{
#ifdef CONFIG_SYSCTL

View File

@ -23,9 +23,6 @@
#include <linux/projid.h>
#include <linux/fs_struct.h>
extern bool setup_userns_sysctls(struct user_namespace *ns);
extern void retire_userns_sysctls(struct user_namespace *ns);
static struct kmem_cache *user_ns_cachep __read_mostly;
static DEFINE_MUTEX(userns_state_mutex);
@ -34,6 +31,7 @@ static bool new_idmap_permitted(const struct file *file,
struct uid_gid_map *map);
static void free_user_ns(struct work_struct *work);
static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
{
/* Start with the same capabilities as init but useless for doing
@ -68,8 +66,12 @@ int create_user_ns(struct cred *new)
kgid_t group = new->egid;
int ret;
ret = -EUSERS;
if (parent_ns->level > 32)
return -EUSERS;
goto fail;
if (!inc_user_namespaces(parent_ns))
goto fail;
/*
* Verify that we can not violate the policy of which files
@ -77,26 +79,27 @@ int create_user_ns(struct cred *new)
* by verifing that the root directory is at the root of the
* mount namespace which allows all files to be accessed.
*/
ret = -EPERM;
if (current_chrooted())
return -EPERM;
goto fail_dec;
/* The creator needs a mapping in the parent user namespace
* or else we won't be able to reasonably tell userspace who
* created a user_namespace.
*/
ret = -EPERM;
if (!kuid_has_mapping(parent_ns, owner) ||
!kgid_has_mapping(parent_ns, group))
return -EPERM;
goto fail_dec;
ret = -ENOMEM;
ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
if (!ns)
return -ENOMEM;
goto fail_dec;
ret = ns_alloc_inum(&ns->ns);
if (ret) {
kmem_cache_free(user_ns_cachep, ns);
return ret;
}
if (ret)
goto fail_free;
ns->ns.ops = &userns_operations;
atomic_set(&ns->count, 1);
@ -106,6 +109,7 @@ int create_user_ns(struct cred *new)
ns->owner = owner;
ns->group = group;
INIT_WORK(&ns->work, free_user_ns);
ns->max_user_namespaces = INT_MAX;
/* Inherit USERNS_SETGROUPS_ALLOWED from our parent */
mutex_lock(&userns_state_mutex);
@ -126,7 +130,11 @@ int create_user_ns(struct cred *new)
key_put(ns->persistent_keyring_register);
#endif
ns_free_inum(&ns->ns);
fail_free:
kmem_cache_free(user_ns_cachep, ns);
fail_dec:
dec_user_namespaces(parent_ns);
fail:
return ret;
}
@ -163,6 +171,7 @@ static void free_user_ns(struct work_struct *work)
#endif
ns_free_inum(&ns->ns);
kmem_cache_free(user_ns_cachep, ns);
dec_user_namespaces(parent);
ns = parent;
} while (atomic_dec_and_test(&parent->count));
}